Implements additional unit tests for integral_flux_image in spectral_cube
Fixes mistake in angle units definitin in image utils.py and affected npred_general tutorial script
Merge with master
Updates spectral cube test in line with angle unit corrections
Minor corrections to npred_general.py and test_spectral_cube.py
Update
Merge pull request #188 from ellisowen/issue_178_test

Add more unit tests for spectral_cube.integral_flux_image
Adds iterative background kernel estimation code to gammapy.background
Adds unit tests for new iterative background kernel estimation code
Updates background init.py file to include background estimation code
Adds background iteratative estimation tutorial RST file
Updates tutorials index.rst file to include new background tutorial
Bug fixes in background kernel script

Bug fixes in background kernel script and updates the tutorial and unit tests